1. Field of the Invention
The present invention relates to a zoom lens barrel and particularly to a focusing method thereof, specifically to a so-called inner focus or rear focus method, in which a plurality of lens groups are moved in a lens system.
2. Related Background Art
There are recently investigated or proposed various focusing methods other than the front lens advance method, for size reduction or the pursuit of high performance of a zoom lens. It is known that the so-called floating-type inner focus or rear focus method with a plurality of lens groups being moved in the lens system is effective especially to achieve high performance. Actually, there are a variety of methods proposed in optical design, but it has been difficult to realize such methods with simple structure in mechanical design together with the so-called manual focus. The reason is that for the so-called floating a plurality of lens groups move independent of each other for focusing and that movement amounts thereof for focusing change with change of zooming position. This made it very difficult to achieve the mechanical structure to attain the movement amount necessary for each focusing lens group at each focusing position and at each zooming position.